Aerial tramways, such as chairlifts, gondolas and the like, are conventionally supported from towers carrying rotating sheave assemblies for guiding and supporting of a wire haul rope to which the chairs, gondolas, etc., are secured. As used herein, the expression "aerial tramway" shall include chairlifts, gondolas, ski lifts, trams and other personnel conveying systems which are based upon a wire haul rope.
Typically, an aerial tramway tower includes a pair of rocker arm assemblies to which the rotatable sheaves are connected for supporting the wire haul rope. A typical rocker arm assembly includes a pair of sheaves mounted near the opposite ends of the rocker arm, with the middle of the arm being pivotally mounted to a support structure. In larger rocker arm assemblies there are four rocker arms on which eight sheaves are rotatably mounted. The four rocker arms are supported from pairs of intermediate rocker arms, and the pair of intermediate rocker arms supported from a common base or support rocker arm. The base or support rocker arm is, in turn, pivotally mounted to a transversely-extending arm from the tower of the aerial tramway. My U.S. Pat. No. 4,462,314 describes such rocker arm assemblies in more detail.
In such aerial tramway systems, it is well known to provide structures to catch the wire haul rope in the event that it runs off a side of one of the sheaves. Such a condition of the haul rope becoming detrained from the sheave is referred to in the art as "deropement". Although it is important to catch the haul rope in the event of a complete or partial deropement, it is even more important to shutdown the rope drive mechanism immediately in the event of deropement and preferably before deropement.
Continued advancement of a deroped haul rope, even if it has been caught by a rope catcher, will advance the passenger carrier units of the tramway dangerously close to the towers. Moreover, if the signal to shut the tramway down is not initiated until after the rope jumps the sheaves, the inertia of the tramway will almost certainly cause at least some of the passenger carrier units to reach the tower at which deropement has occurred, with attendant risk to passengers.
One system for stopping the haul rope drive mechanism in the event of a deropement is shown in my prior U.S. Pat. No. 3,822,369 in which a frangible sensing device is placed proximate a sheave assembly in a position at which a deropement would cause rupture of the frangible sensor.
Another system for detecting a deropement is shown in my U.S. Pat. No. 4,019,002. According to that patent, movable, low inertia portions of the sheave assembly are biased by a spring loaded mechanism assembly for movement in the event of deropement of the haul rope. Although the apparatus of my U.S. Pat. No. 4,019,002 had advantages over the prior art, one disadvantage was its susceptibility to being rendered inoperative because of ice or snow.
Also in the prior art, my U.S. Pat. No. 4,363,945 taught an electrical circuit having a plug-end connector which was mechanically pulled apart from the remainder of the circuit upon deropement of the haul rope. Upon the plug-end connector being pulled apart, the drive system for the haul rope was interrupted or deactivited.
As still another example of the prior art, my U.S. Pat. No. 4,462,314 taught a haul rope deropement detector including a frangible element mounted at a position proximate both the leading and trailing sheaves of a rocker arm assembly. More particularly, the frangible element was placed in a position whereby it would be broken upon deropement of the haul rope onto a rope catcher, and the system operated analogously to a fuse in an electrical circuit to cause interruption of electrical power in the event of a deropement.
All such prior art systems are based upon the haul rope leaving or jumping from the sheave assembly. They are, in effect, after-the-fact shut down systems which require deropement to operate. It is highly desirable, however, to detect an impending deropement and to be able to stop the tramway prior to the rope leaving the sheaves.
One approach to this goal which has been employed in the industry is to provide a proximity switch next to the haul rope. Such proximity switches have taken the form of a reed switch which is normally closed as a result of being attracted to the haul rope. The reed switch is mounted in close proximity to the haul rope as it is supported for axial movement along a normal or nominal path over the sheaves. If the haul rope should be laterally displaced relative to the reed switch, for example by walking or creeping of the haul rope up the side of a sheave prior to deropement, the proximity reed switch will open, shutting down the tramway prior to deropement or at least soon after deropement.
The problem with such prior art reed switch deropement detectors has been that the aerial tramways regularly are subjected to considerable static electrical charges. Such changes can affect the reliability of the opening of the reed switch. Additionally, lightning strikes also are quite common at tramway installations, and the reed proximity switch tends to become contact welded in a closed condition. Thus, when the haul rope deropes from the sheaves, the welded closed reed switch will not open, and the tramway continues to operate. As a result of such problems, the reed-based proximity switch has been banned from use in tramway systems in many European countries.